The current paths in electronic assemblies that include processors are continually being required to handle ever-increasing amounts of current in order to power the processors. Processors typically require more power in order to operate at higher frequencies and to simultaneously perform numerous logic and memory operations. Each increase in processing speed and power generally carries a cost of increased heat generation within electronic assemblies that include processors. As processor power densities continue to increase, so too does the structural and thermal challenge of adhering electronic packages that include processors to a motherboard.
One example method of attaching an electronic package to a motherboard includes soldering the electronic package to the motherboard. One drawback with soldering the electronic package to the motherboard is that the various components contract at different rates during bonding due to differences in the coefficients of thermal expansion for the materials that form the electronic package, solder and motherboard. Since the electronic package, solder and motherboard contract at different rates, stress forms within the electronic package, solder and motherboard as the solder hardens to bond the electronic package to the motherboard.
Most conventional solders have re-flow temperatures around 183° C. and above. This relatively high re-flow temperature leads to significant temperature changes within the electronic package, solder and motherboard as the solder is re-flowed to connect an electronic package to a motherboard. The large temperature change generates significant expansion and contraction within the electronic package, solder and motherboard as the solder is re-flowed and then cooled. The significant expansion and contraction causes stress within the components as the solder hardens. The stress within the components makes the electronic assemblies that include such components vulnerable to unwanted cracking.
One example relates to when a motherboard is mounted within a chassis that is shipped to an end user. The electronic package, solder and motherboard within such electronic assemblies are under stress such that they are particularly vulnerable to the shock and vibration forces generated during shipping.
In addition, the high re-flow temperature of some solders is simply not acceptable for many heat-sensitive devices (e.g., optoelectronic devices). Many electronic assemblies require a re-flow temperature that is less than 125° C. in order to connect an electronic package to a motherboard.
Devices that require a low re-flow temperatures leave only a small thermal window for assembly, as many devices operate at a working temperature around 80° C. The small thermal window is problematic because there are a limited number of solders that have such a low melting temperature (Tm). In addition, many low Tm solders also include an undesirable toxic element (e.g., cadmium).
There are some conventional solders that have a Tm around 100° C. or less. Solders with such low Tm typically cannot be used in most electronic assemblies because their Tm is too close to, or below, the working temperature of many electronic assemblies.
One available alternative is to use adhesives to attach an electronic package to a motherboard. However, adhesives are limited in both thermal and electrical conductivity. Solder alloys are desirable because of their relatively high electrical and thermal conductivities.